Study of 25 dogs and 40-50 neonatal infants to evaluate and compare two techniques for nearly continuous blood gas monitoring of oxygen (PO2) and carbon dioxide (PCO2). In vitro testing of the systems for precision, accuracy and stability, and in vivo testing will be made with dogs. At the conclusion of the in vivo tests, there will be measurements made in the neonatal intensive care unit. Both techniques involve insertion of a catheter into the umbilical artery of newborn infants. The best two of four catheters of their design, plus all commercially available catheters, will be used with a high performance mass spectrometer system. This will be compared with an intermittent sampling system in which gas from a loop catheter system is analyzed in a gas chromatograph every two to three minutes.